<%@ WebHandler Language="C#" Class="verify" %>

using System;

using System.Web;

using System.Text;

using System.IO;

using System.Drawing;

using System.Drawing.Imaging;

public class verify : IHttpHandler, System.Web.SessionState.IRequiresSessionState

{

    public void ProcessRequest(HttpContext context)

    {

        Code gif = new Code();//初始化验证码生成类

        string valid = "";//定义随机数

        MemoryStream ms = gif.Create(out valid);//获取包括验证码图片的内存流

        context.Session["gif"] = valid;//验证码存储在Session中，供验证。

        context.Response.ClearContent();//清空输出流

        context.Response.ContentType = "image/png";//输出流的格式

        context.Response.BinaryWrite(ms.ToArray());//输出

        context.Response.End();

    }

    public bool IsReusable

    {

        get

        {

            return false;

        }

    }  

}

public class Code

{

    /// <summary>

    /// 该方法用于生成指定位数的随机数

    /// </summary>

    /// <param name="VcodeNum">参数是随机数的位数</param>

    /// <returns>返回一个随机数字符串</returns>

    private string RndNum(int VcodeNum)

    {

        //验证码可以显示的字符集合

        //string Vchar = "1,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,i,j,k,l,m,n,p,p" +

        //    ",q,r,s,t,u,v,w,x,y,z,A,B,C,D,E,F,G,H,I,J,K,L,M,N,P,P,Q" +

        //    ",R,S,T,U,V,W,X,Y,Z";

        string Vchar = "1,2,3,4,5,6,7,8,9";//=================================================这里使用纯数字
string[] VcArray = Vchar.Split(new Char[] { ',' });//拆分成数组

        string VNum = "";//产生的随机数

        int temp = -;//记录上次随机数值，尽量避避免生产几个一样的随机数  

        Random rand = new Random();

        //采用一个简单的算法以保证生成随机数的不同

        for (int i = ; i < VcodeNum + ; i++)

        {

            if (temp != -)

            {

                rand = new Random(i * temp * unchecked((int)DateTime.Now.Ticks));//初始化随机类

            }

            //int t = rand.Next(61);//获取随机数

            int t = rand.Next();//======================================================与VcArray定义的长度匹配
if (temp != - && temp == t)

            {

                return RndNum(VcodeNum);//如果获取的随机数重复，则递归调用

            }

            temp = t;//把本次产生的随机数记录起来

            VNum += VcArray[t];//随机数的位数加一

        }

        return VNum;

    }

    /// <summary>

    /// 该方法是将生成的随机数写入图像文件

    /// </summary>

    /// <param name="VNum">VNum是一个随机数</param>

    public MemoryStream Create(out string VNum)

    {

        VNum = RndNum();

        Bitmap Img = null;

        Graphics g = null;

        MemoryStream ms = null;

        System.Random random = new Random();

        //验证码颜色集合

        Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };

        //验证码字体集合

        string[] fonts ={ "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };

        //定义图像的大小，生成图像的实例

        Img = new Bitmap((int)VNum.Length * , );

        g = Graphics.FromImage(Img);//从Img对象生成新的Graphics对象    

        g.Clear(Color.White);//背景设为白色  

        //在随机位置画背景点

        for (int i = ; i < ; i++)

        {

            int x = random.Next(Img.Width);

            int y = random.Next(Img.Height);

            g.DrawRectangle(new Pen(Color.LightGray, ), x, y, , );

        }

        //验证码绘制在g中

        for (int i = ; i < VNum.Length; i++)

        {

            int cindex = random.Next();//随机颜色索引值

            int findex = random.Next();//随机字体索引值

            Font f = new System.Drawing.Font(fonts[findex], , System.Drawing.FontStyle.Bold);//字体======================字体大小

            Brush b = new System.Drawing.SolidBrush(c[cindex]);//颜色

            int ii = ;

            if ((i + ) %  == )//控制验证码不在同一高度

            {

                ii = ;

            }

            g.DrawString(VNum.Substring(i, ), f, b,  + (i * ), ii);//绘制一个验证字符

        }

        ms = new MemoryStream();//生成内存流对象

        Img.Save(ms, ImageFormat.Jpeg);//将此图像以Png图像文件的格式保存到流中  

        //回收资源

        g.Dispose();

        Img.Dispose();

        return ms;

    }

} 

<asp:TextBox ID="txtVerify" runat="server" MaxLength="4" Width="100px" CssClass="txtin2" Height="18px"></asp:TextBox>
<img src='verify.ashx' style="width:100px;height:30px" id="verify"/>
<a href='javascript:verify();'>看不清，换一张</a>

function verify(){

    $("#verify").attr("src","verify.ashx");

}

if (txtVerify.Text == "")

        {

            this.ClientScript.RegisterStartupScript(this.GetType(), "scriptKey", "alert('验证码不能为空！')", true);

        }

        else if (String.Compare(Session["gif"].ToString(), txtVerify.Text, true) != )

        {

            txtVerify.Text = "";

            this.ClientScript.RegisterStartupScript(this.GetType(), "scriptKey", "alert('验证码有误,请重新输入正确的验证码！')", true);

        }

        else

        {

            //通过验证

}